This equation allows you to set a more accurate value of the PP. Typically, the membrane is a few mV less polarized than the equilibrium potential for K + .

Action potential (AP) may occur in excitable cells. If a nerve or muscle is irritated above the excitation threshold, then the RI of the nerve or muscle will quickly decrease and for a short period of time (millisecond) there will be a short-term recharge of the membrane: its inner side will become positively charged relative to the outer, after which the RI will be restored. This short-term change in the PP, which occurs when the cell is excited, is called the action potential.

The occurrence of PD is possible due to the fact that, unlike potassium ions, sodium ions are far from equilibrium. If we substitute sodium instead of potassium into the Nernst equation, we get an equilibrium potential of about +60 mV. During PD, there is a transient increase in Na+ permeability. At the same time, sodium will begin to penetrate into the cell under the action of two forces: along the concentration gradient and along the membrane charge, trying to adjust the membrane charge to its equilibrium potential. The movement of sodium is carried out along potential dependent sodium channels, which open in response to a shift in the membrane potential, after which they themselves are inactivated.

Rice. 2. Action potential nerve fiber(A) and change in membrane conductivity for sodium and potassium ions (B).

On the record, PD looks like a short-term peak (Fig. 2) with several phases.

1. Depolarization (rising phase) (Fig. 2) - an increase in sodium permeability due to the opening of sodium channels. Sodium tends to its equilibrium potential, but does not reach it, since the channel has time to become inactivated.
2. Repolarization is the return of the charge to the value of the resting potential. In addition to the potassium channels of the leak, voltage-dependent potassium channels are connected here (activated by depolarization). At this time, potassium leaves the cell, returning to its equilibrium potential.
3. Hyperpolarization (not always) - occurs in cases where the equilibrium potential for potassium exceeds the modulus of PP. The return to the PP occurs after the return to the equilibrium potential for K + .

During PD, the polarity of the membrane charge changes. The PD phase in which the membrane charge is positive is called overshot(Fig. 2).

The system of activation and inactivation is very important for AP generation. voltage-gated sodium channels(Fig. 3). These channels have two doors: activation (M-gate) and inactivation (H-gate). At rest, the M-gate is open and the H-gate is closed. During membrane depolarization, the M gate opens rapidly and the H gate begins to close. The flow of sodium into the cell is possible while the M-gate is already open, and the H-gate has not yet closed. The entry of sodium leads to further depolarization of the cell, leading to the opening of more channels and starting a chain of positive feedback. Membrane depolarization will continue until all voltage-gated sodium channels are inactivated, which occurs at the peak of AP. The minimum amount of stimulus leading to the occurrence of AP is called threshold. Thus, the emerging AP will obey the all-or-nothing law and its value will not depend on the magnitude of the stimulus that caused the AP.

Due to the H-gate, channel inactivation occurs before the potential on the membrane reaches the equilibrium value for sodium. After the cessation of sodium entry into the cell, repolarization occurs due to potassium ions leaving the cell. At the same time, potential-activated potassium channels are also connected to the leakage channels in this case. During repolarization, the M-gate closes rapidly in the fast sodium channel. The H-gate opens much more slowly and remains closed for some time after the charge returns to the resting potential. This period is called refractory period.

Rice. 3. Operation of the voltage-gated sodium channel.

The concentration of ions inside the cell is restored by sodium-potassium ATPase, which, using energy in the form of ATP, pumps 3 sodium ions out of the cell and pumps 2 potassium ions.

On unmyelinated fiber or along the muscle membrane, the action potential propagates continuously. The resulting action potential due to the electric field is able to depolarize the membrane of the neighboring area to a threshold value, resulting in depolarization in the neighboring area. The main role in the emergence of a potential in a new section of the membrane is the previous section. At the same time, at each site, immediately after the AP, a period of refractory occurs, due to which the AP propagates unidirectionally. Ceteris paribus, the propagation of the action potential along the unmyelinated axon occurs the faster, the larger the fiber diameter. In mammals, the speed is 1-4 m / s. Since invertebrates lack myelin, the AP speed in giant squid axons can reach 100 m/s.

By myelinated fiber The action potential propagates spasmodically (saltatory conduction). Myelinated fibers are characterized by a concentration of voltage-gated ion channels only in the areas of Ranvier intercepts; here their density is 100 times greater than in the membranes of unmyelinated fibers. There are almost no voltage-gated channels in the area of ​​myelin couplings. The action potential that arose in one interception of Ranvier, due to the electric field, depolarizes the membrane of neighboring interceptions to a threshold value, which leads to the emergence of new action potentials in them, that is, excitation passes abruptly, from one interception to another. In the event of damage to one node of Ranvier, the action potential excites the 2nd, 3rd, 4th, and even 5th, since the electrical insulation created by the myelin sleeves reduces the dissipation of the electric field. Saltatory conduction increases the speed of AP conduction 15-20 times up to 120 m/s.

https://shishadrugs.com The work of neurons

The nervous system is made up of neurons and glial cells. However, leading role Neurons play a role in the conduction and transmission of nerve impulses. They receive information from many cells along the dendrites, analyze it and transmit it or not to the next neuron.

The transmission of a nerve impulse from one cell to another is carried out with the help of synapses. There are two main types of synapses: electrical and chemical (Fig. 4). The task of any synapse is to transmit information from presynaptic membrane(axon membrane) on postsynaptic(membrane of a dendrite, other axon, muscle, or other target organ). Most synapses of the nervous system are formed between the end of axons and dendrites, which form dendritic spines in the area of ​​the synapse.

Advantage electrical synapse is that the signal from one cell to another passes without delay. In addition, such synapses do not get tired. To do this, pre- and postsynaptic membranes are connected by transverse bridges through which ions from one cell can move to another. However, a significant disadvantage of such a system is the lack of unidirectional transmission of PD. That is, it can be transmitted both from the presynaptic membrane to the postsynaptic one, and vice versa. Therefore, such a design is quite rare and mainly - in nervous system invertebrates.

Rice. 4. Scheme of the structure of chemical and electrical synapses.

chemical synapse very common in nature. O is more complicated, since a system is needed for converting an electrical impulse into a chemical signal, then again into an electrical impulse. All this gives rise to synaptic delay, which can be 0.2-0.4 ms. In addition, stock depletion may occur. chemical which will lead to synapse fatigue. However, such a synapse provides unidirectional transmission of AP, which is its main advantage.

Rice. Fig. 5. Scheme of work (a) and electron micrograph (b) of a chemical synapse.

At rest, the end of the axon, or presynaptic terminal, contains membrane vesicles (vesicles) with a neurotransmitter. The surface of the vesicles is negatively charged to prevent binding to the membrane and coated with special proteins involved in the release of the vesicles. Each vial contains the same amount of a chemical called quantum neurotransmitter. Neurotransmitters are very diverse chemical structure, however, most of them are produced right at the end. Therefore, it may contain systems for the synthesis of a chemical mediator, as well as the Golgi apparatus and mitochondria.

postsynaptic membrane contains receptors to the neurotransmitter. Receptors can be in the form of ion channels that open upon contact with their ligand ( ionotropic), and membrane proteins that trigger an intracellular cascade of reactions ( metabotropic). One neurotransmitter can have several ionotropic and metabotropic receptors. At the same time, some of them can be excitatory, and some - inhibitory. Thus, a cell's response to a neurotransmitter will determine the type of receptor on its membrane, and different cells can react quite differently to the same chemical.

Between the pre- and postsynaptic membrane is located synaptic cleft, 10-15 nm wide.

When AP arrives at the presynaptic ending, voltage-activated calcium channels open on it and calcium ions enter the cell. Calcium binds to proteins on the surface of the vesicles, which leads to their transport to the presynaptic membrane, followed by membrane fusion. After such an interaction, the neurotransmitter finds itself in the synaptic cleft (Fig. 5) and can bind to its receptor.

Ionotropic receptors are ligand-activated ion channels. This means that the channel only opens in the presence of a certain chemical. For different neurotransmitters, these can be sodium, calcium, or chloride channels. The current of sodium and calcium causes membrane depolarization, therefore, such receptors are called excitatory. Chlorine current leads to hyperpolarization, which makes it difficult to generate AP. Therefore, such receptors are called inhibitory.

Metabotropic neurotransmitter receptors belong to the class of G protein-associated receptors (GPCRs). These proteins trigger a variety of intracellular cascades of reactions that ultimately lead to either further transmission of excitation or inhibition.

After signal transmission, it is necessary to quickly remove the neurotransmitter from the synaptic cleft. For this, either enzymes that decompose a neurotransmitter are present in the gap, or transporters pumping the mediator into the cells can be located on the presynaptic ending or neighboring glial cells. In the latter case, it can be reused.

Each neuron receives impulses from 100 to 100,000 synapses. A single depolarization on one dendrite will not result in further signal transmission. A neuron can receive both excitatory and inhibitory stimuli simultaneously. All of them summed up on the soma of the neuron. This summation is called spatial. Further, PD may or may not occur (depending on the incoming signals) in the area axon colliculus. The axon hillock is the area of ​​the axon adjacent to the soma and has a minimum AP threshold. Further, the impulse propagates along the axon, the end of which can strongly branch and form synapses with many cells. In addition to the spatial, there is time summation. It occurs in the case of the receipt of frequently repeated impulses from one dendrite.

In addition to classical synapses between axons and dendrites or their spines, there are also synapses that modulate transmission in other synapses (Fig. 6). These include axo-axonal synapses. Such synapses are able to enhance or inhibit synaptic transmission. That is, if an AP arrives at the end of the axon that forms the axo-spinous synapse, and at that time an inhibitory signal arrives at it via the axo-axonal synapse, the release of the neurotransmitter in the axo-spinous synapse will not occur. Axodendritic synapses can change the conduction of AP by the membrane on the way from the spine to the cell soma. There are also axo-somatic synapses that can affect signal summation in the region of the soma of the neuron.

Thus, there is a huge variety of different synapses, differing in the composition of neurotransmitters, receptors and their location. All this provides a variety of reactions and plasticity of the nervous system.

Rice. 6. Variety of synapses in the nervous system.

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resting membrane potential (MPP) or resting potential (PP) is the potential difference of a resting cell between the inner and outer sides of the membrane. The inner side of the cell membrane is negatively charged relative to the outer. Taking the potential of the external solution as zero, the MPP is recorded with a minus sign. Value WFP depends on the type of tissue and varies from -9 to -100 mV. Therefore, at rest, the cell membrane polarized. A decrease in the MPP value is called depolarization increase - hyperpolarization, restoring the original value WFP- repolarization membranes.

The main provisions of the membrane theory of origin WFP come down to the following. At rest, the cell membrane is well permeable to K + ions (in some cells and to SG), less permeable to Na + and practically impermeable to intracellular proteins and other organic ions. K + ions diffuse out of the cell along a concentration gradient, while non-penetrating anions remain in the cytoplasm, providing the appearance of a potential difference across the membrane.

The resulting potential difference prevents the exit of K + from the cell, and at a certain value, an equilibrium occurs between the exit of K + along the concentration gradient and the entry of these cations along the resulting electrical gradient. The membrane potential at which this equilibrium is reached is called equilibrium potencyscarlet Its value can be calculated from the Nernst equation:

where E to- equilibrium potential for To + ; R- gas constant; T- absolute temperature; F - Faraday number; P- valency K + (+1), [K n +] - [K + vn] - external and internal concentrations of K + -

If we switch from natural logarithms to decimal logarithms and substitute the numerical values ​​of the constants into the equation, then the equation will take the form:

In spinal neurons (Table 1.1) E k = -90 mV. The MPP value measured using microelectrodes is noticeably lower, 70 mV.

Table 1.1. The concentration of some ions inside and outside the spinal motor neurons of mammals

If the membrane potential of a cell is of a potassium nature, then, in accordance with the Nernst equation, its value should decrease linearly with a decrease in the concentration gradient of these ions, for example, with an increase in the concentration of K + in the extracellular fluid. However linear dependence the magnitude of the MPP (membrane resting potential) from the concentration gradient of K + exists only when the concentration of K + in the extracellular fluid is above 20 mm. At lower concentrations of K + outside the cell, the dependence curve of E m on the logarithm of the ratio of potassium concentration outside and inside the cell differs from the theoretical one. It is possible to explain the established deviations of the experimental dependence of the MPP value and the K + concentration gradient theoretically calculated by the Nernst equation by assuming that the MPP of excitable cells is determined not only by potassium, but also by sodium and chloride equilibrium potentials. Arguing similarly to the previous one, we can write:

The values ​​of sodium and chloride equilibrium potentials for spinal neurons (Table 1.1) are +60 and -70 mV, respectively. The value of E Cl is equal to the value of the MPP. This indicates a passive distribution of chloride ions through the membrane in accordance with chemical and electrical gradients. For sodium ions, the chemical and electrical gradients are directed inside the cell.

The contribution of each of the equilibrium potentials to the MPP value is determined by the ratio between the permeability of the cell membrane for each of these ions. The membrane potential value is calculated using the Goldman equation:

E m- membrane potential; R- gas constant; T- absolute temperature; F- Faraday number; RK, P Na and RCl- membrane permeability constants for K + Na + and Cl, respectively; [TO+ n ], [ K + ext, [ Na+ n [ Na + ext], [Cl - n] and [Cl - ext] - concentrations of K + , Na + and Cl outside (n) and inside (ext) of the cell.

Substituting into this equation the ion concentrations and the MPP value obtained in experimental studies, it can be shown that for the giant squid axon there should be the following ratio of the permeability constants P to: P Na: P C1 = I: 0.04: 0.45. Obviously, since the membrane is permeable to sodium ions (P N a =/ 0) and the equilibrium potential for these ions has a plus sign, then the entry of the latter into the cell along the chemical and electrical gradients will reduce the electronegativity of the cytoplasm, i.e. increase the MPP (membrane resting potential).

With an increase in the concentration of potassium ions in the external solution above 15 mM, the MPP increases and the ratio of the permeability constants changes towards a more significant excess of Pk over P Na and P C1. P c: P Na: P C1 = 1: 0.025: 0.4. Under such conditions, the MPP is determined almost exclusively by the gradient of potassium ions; therefore, the experimental and theoretical dependences of the MPP on the logarithm of the ratio of potassium concentrations outside and inside the cell begin to coincide.

Thus, the presence of a stationary potential difference between the cytoplasm and the external environment in a resting cell is due to the existing concentration gradients for K + , Na + and Cl and different membrane permeability for these ions. The main role in the generation of MPP is played by the diffusion of potassium ions from the cell into the outer lumen. Along with this, the MPP is also determined by the sodium and chloride equilibrium potentials, and the contribution of each of them is determined by the relationship between the permeabilities plasma membrane cells for these ions.

All the factors listed above constitute the so-called ionic component RMP (membrane resting potential). Since neither potassium nor sodium equilibrium potentials are equal to MPP. the cell must absorb Na + and lose K + . The constancy of the concentrations of these ions in the cell is maintained by the work of Na + K + -ATPase.

However, the role of this ion pump is not limited to maintaining sodium and potassium gradients. It is known that the sodium pump is electrogenic and during its operation a net flow of positive charges arises from the cell into the extracellular fluid, which causes an increase in the electronegativity of the cytoplasm with respect to the environment. The electrogenicity of the sodium pump was revealed in experiments on giant mollusk neurons. Electrophoretic injection of Na + ions into the body of a single neuron caused membrane hyperpolarization, during which the MPP was significantly lower than the potassium equilibrium potential. This hyperpolarization was weakened by lowering the temperature of the solution in which the cell was located, and was suppressed by the specific inhibitor of Na + , K + -ATPase ouabain.

From what has been said, it follows that the MPP can be divided into two components - "ionic" and "metabolic". The first component depends on the concentration gradients of ions and membrane permeabilities for them. The second, "metabolic", is due to the active transport of sodium and potassium and has a dual effect on MPP. On the one hand, the sodium pump maintains concentration gradients between the cytoplasm and the environment. On the other hand, being electrogenic, the sodium pump exerts direct influence at the MPP. Its contribution to the MPP value depends on the density of the “pumping” current (current per unit area of ​​the cell membrane surface) and the membrane resistance.

Membrane action potential

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If a nerve or muscle is irritated above the excitation threshold, then the MPP of the nerve or muscle will quickly decrease and for a short period of time (millisecond) the membrane will be recharged: its inner side will become positively charged relative to the outer. This is a short-term change in the MPP that occurs when the cell is excited, which has the form of a single peak on the oscilloscope screen, is called membrane action potential (MPD).

IVD in the nervous and muscle tissues occurs when the absolute value of the MPP (membrane depolarization) decreases to a certain critical called generation threshold MTD. In the giant nerve fibers of the squid, the MPD is -60 mV. When the membrane is depolarized to -45 mV (the IVD generation threshold), IVD occurs (Fig. 1.15).

During IVD initiation in the squid axon, the membrane resistance decreases by a factor of 25, from 1000 to 40 Ohm.cm2, while the capacitance does not change. This decrease in membrane resistance is due to an increase in the ion permeability of the membrane upon excitation.

In terms of its amplitude (100-120 mV), the MPD (Membrane Action Potential) is 20-50 mV higher than the value of the MPP (Resting Membrane Potential). In other words, the inner side of the membrane on a short time becomes positively charged with respect to the outer, - "overshoot" or charge reversal.

It follows from the Goldmann equation that only an increase in the permeability of the membrane for sodium ions can lead to such changes in the membrane potential. The value of Ek is always less than the value of the MPP, so an increase in the permeability of the membrane for K + will increase the absolute value of the MPP. The sodium equilibrium potential has a plus sign, so a sharp increase in the membrane permeability for these cations leads to membrane recharging.

During IVD, the permeability of the membrane to sodium ions increases. Calculations have shown that if at rest the ratio of the membrane permeability constants for K + , Na + and SG is 1:0.04:0.45, then at IVD - Р to: P Na: Р = 1: 20: 0.45 . Consequently, in the state of excitation, the nerve fiber membrane not only loses its selective ion permeability, but, on the contrary, from being selectively permeable to potassium ions at rest, it becomes selectively permeable to sodium ions. An increase in the sodium permeability of the membrane is associated with the opening of voltage-dependent sodium channels.

The mechanism that provides opening and closing of ion channels is called channel gate. It is customary to distinguish activation(m) and inactivation(h) gate. The ion channel can be in three main states: closed (m-gates are closed; h-open), open (m- and h-gates are open) and inactivated (m-gates are open, h-gates are closed) (Figure 1.16).

Rice. 1.16 Scheme of the position of activation (m) and inactivation (h) gates of sodium channels, corresponding to closed (rest, A), open (activation, B) and inactivated (C) states.

Membrane depolarization caused by an irritating stimulus, such as electric shock, opens the m-gate of sodium channels (transition from state A to B) and ensures the appearance of an inwardly directed flow of positive charges - sodium ions. This leads to further depolarization of the membrane, which in turn increases the number of open sodium channels and therefore increases the sodium permeability of the membrane. There is a "regenerative" depolarization of the membrane, as a result of which the potential inside membrane tends to reach the value of the sodium equilibrium potential.

The reason for the cessation of the growth of IVD (Membrane Action Potential) and repolarization of the cell membrane is:

a) Increased membrane depolarization, i.e. when E m -» E Na, as a result of which the electrochemical gradient for sodium ions decreases, equal to E m -> E Na. In other words, the force "pushing" sodium into the cell decreases;

b) Depolarization of the membrane generates the process of inactivation of sodium channels (closing of the h-gate; state of the B channel), which inhibits the growth of sodium permeability of the membrane and leads to its decrease;

in) Depolarization of the membrane increases its permeability to potassium ions. The outgoing potassium current tends to shift the membrane potential towards the potassium equilibrium potential.

Decreasing the electrochemical potential for sodium ions and inactivating sodium channels reduces the amount of incoming sodium current. At a certain point in time, the value of the incoming sodium current is compared with the increased outgoing current - the growth of the MTD stops. When the total outgoing current exceeds the incoming one, membrane repolarization begins, which also has a regenerative character. The repolarization that has begun leads to the closing of the activation gate (m), which reduces the sodium permeability of the membrane, accelerates repolarization, and the latter increases the number of closed channels, etc.

The phase of IVD repolarization in some cells (for example, in cardiomyocytes and a number of smooth muscle cells) can slow down, forming plateau PD, due to complex changes in time of incoming and outgoing currents through the membrane. In the aftereffect of IVD, hyperpolarization and/or depolarization of the membrane may occur. These are the so-called trace potentials. Trace hyperpolarization has a dual nature: ionic and metabolickuyu. The first is related to the fact that the potassium permeability in the nerve fiber of the membrane remains elevated for some time (tens and even hundreds of milliseconds) after IVD generation and shifts the membrane potential towards the potassium equilibrium potential. The trace hyperpolarization after rhythmic stimulation of cells is associated mainly with the activation of the electrogenic sodium pump, due to the accumulation of sodium ions in the cell.

The reason for the depolarization that develops after the generation of the IVD (Membrane Action Potential) is the accumulation of potassium ions in outer surface membranes. The latter, as it follows from the Goldman equation, leads to an increase in the RRP (Resting Membrane Potential).

Associated with sodium channel inactivation important property nerve fiber calledrefractoriness .

During absofierce refractory period the nerve fiber completely loses the ability to be excited by the action of a stimulus of any strength.

Relative refractoriness, following the absolute, is characterized by a higher threshold for the occurrence of IVD (Membrane Action Potential).

The idea of ​​membrane processes occurring during excitation of the nerve fiber serves as the basis for understanding and the phenomenon accommodation. At the basis of tissue accommodation with a small steepness of the rise of the irritating current is an increase in the excitation threshold, which is ahead of the slow depolarization of the membrane. The increase in the excitation threshold is almost entirely determined by the inactivation of sodium channels. The role of an increase in the potassium permeability of the membrane in the development of accommodation is that it leads to a drop in the resistance of the membrane. Due to the decrease in resistance, the rate of membrane depolarization becomes even slower. The rate of accommodation is higher than more sodium channels at the resting potential is in an inactivated state, the higher the rate of development of inactivation and the higher the potassium permeability of the membrane.

Carrying out excitation

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Conduction of excitation along the nerve fiber is carried out due to local currents between the excited and resting sections of the membrane. The sequence of events in this case is presented as follows.

When a point stimulation is applied to a nerve fiber, an action potential arises in the corresponding section of the membrane. The inner side of the membrane at a given point is positively charged with respect to the adjacent, resting side. Between the points of the fiber that have different potentials, a current arises (local current), directed from excited (sign (+) on the inside of the membrane) to unexcited (sign (-) on the inside of the membrane) to the fiber section. This current has a depolarizing effect on the fiber membrane in the resting area, and when the critical level of membrane depolarization is reached in this area, an MPD (Membrane Action Potential) occurs. This process consistently spreads to all parts of the nerve fiber.

In some cells (neurons, smooth muscles), IVD is not of a sodium nature, but is due to the entry of Ca 2+ ions through voltage-dependent calcium channels. In cardiomyocytes, IVD generation is associated with incoming sodium and sodium-calcium currents.

In this topic, two cations will be considered - sodium (Na) and potassium (K). Speaking of anions, let's take into account that a certain amount of anions is located at the outer and inner sides of the cell membrane.

The shape of a cell depends on which tissue it belongs to. In its own way form cells can be

Cylindrical and cubic (skin cells);

discoid (erythrocytes);

spherical (ovules);

fusiform (smooth muscle);

stellate and pyramidal nerve cells);

Not having a permanent form - amoeboid (leukocytes).

The cell has a number properties: it feeds, grows, reproduces, recovers, adapts to its environment, exchanges energy and substances with environment, performs its inherent functions (depending on which tissue the given cell belongs to). In addition, the cell has excitability.

Excitability – This is the ability of a cell to move from a state of rest to a state of activity in response to stimuli.

Irritations can come from external environment or originate within the cell. The stimuli causing excitation can be: electrical, chemical, mechanical, temperature and other stimuli.

A cell can be in two main states - at rest and in excitation. The rest and excitation of the cell is otherwise called - resting membrane potential and membrane action potential.

When the cell does not experience any irritation, it is at rest. Rest of the cell is otherwise called resting membrane potential (RMP).

At rest, the inner surface of its membrane is negatively charged, and the outer is positively charged. This is explained by the fact that there are many anions and few cations inside the cell, while behind the cell, on the contrary, cations predominate.

Since there are electric charges in the cell, the electricity they create can be measured. The value of the resting membrane potential is: - 70 mV, (minus 70, since there is a negative charge inside the cell). This value is conditional, since each cell can have its own value of the resting potential.

At rest, the membrane pores are open to potassium ions and closed to sodium ions. This means that potassium ions can easily enter and leave the cell. Sodium ions cannot enter the cell because the pores of the membrane are closed to them. But a small number of sodium ions enter the cell because they are attracted large quantity anions located on the inner surface of the membrane (opposite charges are attracted). This movement of ions is passive , because it does not require energy.

For normal cell activity, the value of its MPP must remain at a constant level. However, the movement of sodium and potassium ions across the membrane causes fluctuations in this value, which can lead to a decrease or increase in the value: - 70 mV.

In order for the MPP to remain relatively constant, the so-called sodium-potassium pump . Its function is that it removes sodium ions from the cell, and pumps potassium ions into the cell. It is a certain ratio of sodium and potassium ions in the cell and outside the cell that creates the desired value of the MPP. Pump operation is active mechanism , because it requires energy.

The source of energy in the cell is ATP. ATP gives energy only when splitting into a simpler acid - ADP, with the obligatory participation in the reaction of the enzyme ATP-ase:

ATP + enzyme ATPase ADP + energy

Membrane potential (MP) is the potential difference between the outer and inner surfaces of the membrane of an excitable cell at rest. On average, in cells of excitable tissues, the MP reaches 50-80 mV, with a minus sign inside the cell. A study of the nature of the membrane potential showed that in all excitable cells (neurons, muscle fibers, myocardiocytes, smooth muscle cells) its presence is due mainly to K+ ions. As is known, in excitable cells, due to the operation of the Na-K-pump, the concentration of K+ ions in the cytoplasm at rest is maintained at a level of 150 mM, while in the extracellular medium the concentration of this ion usually does not exceed 4–5 mM. This means that the intracellular concentration of K+ ions is 30–37 times higher than the extracellular one. Therefore, along the concentration gradient, K+ ions tend to exit the cell into the extracellular environment. Under resting conditions, indeed, there is a flow of K + ions leaving the cell, while diffusion is carried out along potassium channels, most of which are open. As a result of the fact that the membrane of excitable cells is impermeable to intracellular anions (glutamate, aspartate, organic phosphates), an excess of negatively charged particles is formed on the inner surface of the cell membrane due to the release of K + ions, and on the outer surface - an excess of positively charged particles. A potential difference arises, i.e., a membrane potential, which prevents the excessive release of K + ions from the cell. At a certain value of the magnetic field, an equilibrium occurs between the exit of K+ ions along the concentration gradient and the entry (return) of these ions along the emerging electrical gradient. The membrane potential at which this equilibrium is reached is called the equilibrium potential. In addition to K+ ions, Na+ and Cl ions make a certain contribution to the creation of the membrane potential. In particular, it is known that the concentration of Na+ ions in the extracellular medium is 10 times higher than inside the cell (140 mM versus 14 mM). Therefore, Na+ ions tend to enter the cell at rest. However, most of the sodium channels are closed at rest (the relative permeability for Na+ ions, judging by the experimental data obtained on the giant squid axon, is 25 times lower than for K+ ions). Therefore, only a small flow of Na+ ions enters the cell. But even this is enough to at least partially compensate for the excess of anions inside the cell. The concentration of Cl- ions in the extracellular medium is also higher than inside the cell (125 mM versus 9 mM), and therefore these anions also tend to enter the cell, apparently through chloride channels.

Membrane potential

The resting membrane potential of large nerve fibers, when no nerve signals are conducted through them, is about -90 mV. This means that the potential inside the fiber is 90 mV more negative than the potential of the extracellular fluid outside the fiber. In the following, we will explain all the factors that determine the level of this resting potential, but first it is necessary to describe the transport properties of the nerve fiber membrane for sodium and potassium ions at rest. Active transport of sodium and potassium ions across the membrane. Sodium-potassium pump. Recall that all cell membranes of the body have a powerful Na + / K + -Hacoc, constantly pumping sodium ions out of the cell and pumping potassium ions into it. This is an electrogenic pump, since more positive charges are pumped out than in (3 sodium ions for every 2 potassium ions, respectively). As a result, a general deficit of positive ions is created inside the cell, leading to a negative potential from the inside of the cell membrane. Na+/K+-Hacoc also creates a large concentration gradient for sodium and potassium across the nerve fiber membrane at rest: Na+ (outside): 142 meq/l Na+ (inside): 14 meq/l K+ (outside): 4 meq/l K + (inside): 140 meq/l Accordingly, the ratio of the concentrations of two ions inside and outside is: Na inside / Na outside - 0.1 K inside / -K outside = 35.0

Leakage of potassium and sodium across the nerve fiber membrane. The figure shows a channel protein in the nerve fiber membrane, called the potassium-sodium leak channel, through which potassium and sodium ions can pass. The leakage of potassium is especially significant, since the channels are more permeable to potassium ions than sodium (normally about 100 times). As discussed below, this difference in permeability is extremely important in determining the level of the normal resting membrane potential.

Thus, the main ions that determine the magnitude of the magnetic field are K+ ions leaving the cell. Na+ ions, which enter the cell in small amounts, partially reduce the magnitude of the magnetic field, and Cl- ions, which also enter the cell at rest, to a certain extent compensate for this effect of Na+ ions. By the way, in numerous experiments with various excitable cells, it has been established that the higher the permeability of the cell membrane for Na+ ions at rest, the lower the MF value. In order for the magnetic field to be maintained at a constant level, it is necessary to maintain ionic asymmetry. For this, in particular, ion pumps (Na-K-pump, and probably also Cl-pump) are used, which restore ionic asymmetry, especially after the act of excitation. Since this type of ion transport is active, i.e., requiring energy expenditure, the constant presence of ATP is necessary to maintain the membrane potential of the cell.

The nature of the action potential

Action potential (AP) is a short-term change in the potential difference between the outer and inner surfaces of the membrane (or between two points in the tissue), which occurs at the time of excitation. When registering the action potential of neurons with the help of a microelectrode tech, a typical peak-like potential is observed. In a simplified form, when AP occurs, the following phases can be distinguished: the initial stage of depolarization, then a rapid decrease in the membrane potential to zero and recharging of the membrane, then the initial level of the membrane potential is restored (repolarization). Na+ ions play the main role in these processes; depolarization is initially due to a slight increase in membrane permeability for Na+ ions. But the higher the degree of depolarization, the higher the permeability of sodium channels becomes, the more sodium ions enter the cell and the higher the degree of depolarization. During this period, there is not only a decrease in the potential difference to zero, but also a change in the polarization of the membrane - at the height of the AP peak, the inner surface of the membrane is positively charged relative to the outer one. The processes of repolarization are associated with an increase in the release of K+ ions from the cell through the opened channels. In general, it should be noted that the generation of an action potential is difficult process, which is based on a coordinated change in the permeability of the plasma membrane for two or three main ions (Na +, K + and Ca ++). The main condition for the excitation of an excitable cell is to reduce its membrane potential to a critical level of depolarization (CDL). Any stimulus or agent capable of lowering the membrane potential of an excitable cell to a critical level of depolarization can excite that cell. As soon as the MT reaches the level of the CUD, the process will continue on its own and lead to the opening of all sodium channels, i.e., to the generation of a full-fledged AP. If the membrane potential does not reach this level, then best case there will be a so-called local potential (local response).

In a number of excitable tissues, the value of the membrane potential is not constant over time - it periodically decreases (i.e., spontaneous depolarization occurs) and independently reaches the FCA, resulting in spontaneous excitation, after which the membrane potential is restored to its original level, and then the cycle repeats. This property is called automation. However, the excitation of most excitable cells requires the presence of an external (in relation to these cells) stimulus.